Process for producing shaped steel parts

ABSTRACT

A process for producing a shaped part of a pre-form of hardenable and heat formable steel sheet, by means of heat treatment with cold deforming tools, in which by the heat forming a hardened component is produced, which exhibits a martensitic and/or banitic microstructure, wherein the hardened component is tempered subsequent to heat forming so that a shaped part is formed which exhibits, at least in areas, a higher yield strength and a pronounced break limit compared to the hardened component.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention concerns a process for producing a shaped component from apre-form or semi-manufactured part of hardenable and hot formable steelsheet metal, by means of hot working with cold deforming tools, whereinas a result of the hot forming a hardened component is produced, whichexhibits a martensitic and/or bainitic microstructure. The invention inparticular concerns a process for press-hardening of steel sheets forproduction of automobile parts with high toughness.

2. Related Art of the Invention

For weight reduction in body construction and in body components thecurrent trend is to reduce sheet metal thickness. For this, it isnecessary that the conventional steels are replaced with hard and hardlyhardened steel materials. The savings in materials must be compensatedfor by an increased toughness of the materials. With respect to therequirements of tensile strength, steel 22MnB5 is suited for example,which belongs to the class of the ultra highly hardened steels with astiffness of 2000 MPa and more.

Press hardening is a particularly economical process for the seriesproduction of shaped components. This applies in particular forsheet-like pre-forms or semi-manufactured parts, since here, due to thelow thickness, a through-hardening of the entire sheet is made possible.Press hardening is known for example from DE 198 15 022 A1, in which thepre-form is first cold formed by a drawing process, thereupon heated andthen press hardened in a hot forming tool.

From DE 197 43 802 C1 a process for press hardening of sheet blanks isknown. Therein a metallic shaped part for vehicle body components, whichhas areas exhibiting a higher ductility, is produced from aboron-manganese steel alloy. The composition of the steel, expressed itin weight percent, is: carbon (C) 0.18% to 0.3%, silicon (Si) 0.1% to0.7%, manganese (Mn) 1.0% to 2.5%, phosphorus (P) maximum 0.025%, chrome(Cr) 0.1% to 0.8%, molybdenum (Mo) 0.1% to 0.5%, sulfur (S) maximum0.1%, titanium (Ti) 0.02% to 0.05%, boron (B) 0.02% to 0.05%, aluminum(Al) 0.01% to 0.6%. The sheet blanks are heated to temperatures of about900° C. and formed in a cold pressed tool wherein aging occurs.Subsequently individual areas of the shaped component are subjected to apartial thermal treatment at temperatures of between 600° C. and 900° C.

Press hardened components are in general very brittle, so that thepossibility of their employment in motor vehicle bodies, in particularin crash structures, is very limited as long as they are in thiscondition. A material characteristic with a pronounced yield strengthand high elastic limit behavior Re/Rm would be needed to come close tothe requirements of car body manufacturing. Likewise, for the componentdesign for a passenger cell, besides the construction geometry, thecomponent-material-yield strength is also determinative, and not themaximal component-material-tensile strength.

It is thus the task of the invention to provide an economicalmanufacturing process for high strength steel shaped components, inwhich the material exhibits both a high strength as well as a highductility.

SUMMARY OF THE INVENTION

The task is inventively solved by a process for production of a shapedcomponent of a blank or pre-form of hardenable and hot deformable steelsheet metal, by means of hot forming with cold deforming tools, whereinby the hot forming a hardened component is produced, which exhibits amartensitic and/or bainitic microstructure, when the hardened componentis tempered after the hot forming, so that a shaped component isproduced which, in comparison to the hardened component condition,exhibits in areas a higher yield strength.

In a further embodiment, the invention is solved by a process forproduction of a shaped component of a blank or pre-form of hardenableand hot formable steel sheet metal, including the steps:

-   a) heating the semi-finished part to an austenitising temperature    and;-   b) press hardening in a cold tool, wherein by quenching at least    partially a martensitic and/or bainitic microstructure is produced;    and-   c) annealing the deformed semi-finished product at temperatures    below 400° C. with formation of a shaped part with increased yield    strength and/or tensile strength compared to the hardened component.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail withreference to the figures which show in:

FIG. 1. a tension/elongation diagram for component A according to theinvention, and

FIG. 2. a tension/elongation diagram for comparative Component B.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention it is proposed that a hot formed andhardened component is tempered in a cold tool, so that a shaped part isproduced, which in comparison to the hardened component conditionexhibits at least in areas a higher and a pronounced yield strength. Bytempering of the component and improvement of the mechanicalcharacteristics takes place. Therein it is important that the yieldstrength is increased and a pronounced yield strength is achieved.Thereby the component is not only highly hard, but also sufficientlyductile for use in vehicle body construction.

A pronounced yield strength is achieved when the tension-elongationcurve reflecting a draw test clearly deviates following a linear area(Hookish area) and exhibits a more or less pronounced short almost levelprogression. The yield strength (elastic limit) (Rm) is defined by thelocation of the deviation point. In materials without pronounced elasticlimit, frequently the Rp0.2-limit is used for the constructivedetermination or interpretation.

It is further envisioned in accordance with the invention that the presshardened component is subject to a thermal treatment below 400° C.,which leads to a shaped component with higher yield strength and/orbreak elongation in comparison to the hardened component. This thermaltreatment is to be understood as annealing at low temperatures. Thetemperatures selected in accordance with the invention lay significantlybelow the temperature level conventional for annealing high and highstrength steels. By the inventive thermal treatment a component withhigh strength at simultaneously high ductility is produced, which meetsthe requirements of vehicle body construction and in particular also issatisfactory in crash structures in automobiles.

Preferably the temperature and the duration of the tempering orannealing is so selected that the yield strength is increased by atleast 20%. Therein the breaking elongation of the shaped part can alsobe increased or, at worst, be only insubstantially reduced.

In the crash structures in the case of deformation it is desired thatthe components have high energy absorption during deformation. This isachieved by the combination of strength and ductility achievable withthe inventive process.

In a preferred embodiment of the invention the semi-manufactured part isformed of a chrome-molybdenum steel. The content of Cr and Mo preferablylies, for Cr, at 0.8 to 1.3% and, for Mo, at 0.13 to 0.4%, in which thecontent of carbon lies at 0.2 to 0.5%.

Suitable examples of these steels include 25CrMo4, 34CrMo4 or 42CrMo4,as well as 25CrMoS4, 34CrMoS4 and 42CrMoS4. For these steels, themanufacturers recommend hardening temperatures in the range of 840 to880° C. and annealing temperatures at 540 to 680° C. The indicatedannealing temperatures may lead to an improvement in ductility, howeverat the same time they also lead to an unacceptable reduction in strengthor, as the case may be, yield strength. For the inventive temperaturesof annealing below 400° C. no cause is observed for embrittlement, butrather an increase in lateral contraction, yield strength and breakelongation. Tempering is preferably carried out in the temperature rangeof 250° C. to 400° C. For these steels the annealing temperatureparticularly preferably lies in the range of 250 to 350° C.

In a preferred further development of the invention the similarmanufactured component is formed of a boron alloyed case-hardened steelor heat-treated (quenched and tempered) steel. Suitable representativesof these steels are, for example, 17MnB3, 22MnB5, or 27MNCr5-2.Particularly preferably the annealing temperature for these steels liesin the range of 300 to 330° C.

The time required for annealing depends in particular upon the materialthickness of the component. For example, for material thickness of about1 mm, 2 to 10 minutes are suitable. If the holding temperature is toolong then depending upon the steel alloy negative influences can beexercised on the component or, as the case may be, on the materialcharacteristics.

For the inventive manufacturing process sheet-like semi-manufacturedparts are particularly preferably suited. Preferably thesemi-manufactured component is comprised of a flat plate of an alreadycold deformed steel blank or of a cut component semi-manufactured part.The material thickness typically lies in the range of 0.8 to 3 mm.

By the inventive deformation process, in particular profile members orbeams or hollow beams for automobile body and undercarriage areproduced.

It is useful that the hardened pre-form or semi-manufactured componentis cut to the end contour or shape only directly prior to or after theannealing or, as the case may be, tempering. For this, the surface ofthe hardened shaped part is preferably cleaned prior to annealing. Acontamination of this surface during annealing is to be avoided ifpossible, so that the follow up treatment of the surface followingannealing can be minimized. A preferred cleaning process is particleblasting and dry cleaned by means of blasting.

In a further preferred embodiment of the invention a surface treatmentor formation of a defined corrosion protection and/or wear protectionfor the shaped part is carried out during annealing (in step c)),preferably simultaneously also during tempering. Therein the temperatureof the annealing, which can be for example 250 to 400° C., can be takenadvantage of or used for burning in of a coating, in particularcorrosion protection coating.

Particularly preferred is when during annealing or, as the case may be,tempering, a galvanizing or thin layer zinking is carried out as thesurface treatment. Herein the semi-finished product is preferablycomprised of multiple heat treatable steel sheets, of which at least oneis a hardenable steel.

The process is preferably used for manufacture of a form part for ahollow beam integrated in a passenger cell of a motor vehicle body orfor a hollow beam or support member integrated in the undercarriage of avehicle. Particularly preferred is when the hollow beam is provided inthe crash structure or the deformation structure of automobiles.

EXAMPLE

For comparative purposes a number of pre-forms of a 22MnB5-steel weresubjected to an inventive heat formation or, as the case may be, presshardening, thereafter annealing (Component A) and on the other hand onlyhot forming or as the case may be press hardening (Component B). Thematerial thickness of the sheet metal plates lies at approximately 1.0mm.

For annealing (Component A) a temperature of 320° C. and a duration ofapproximately 4 to 6 minutes was selected.

For the Component A the following average values resulted: YieldStrength 1226 MPa Tensile Strength 1335 MPa Tear Strength 1000 MPaStretch Load Limit 2.4% Elongation Breakage 4.1% Contraction at Fracture29.8% 

For component B the following average values resulted: Yield Strength 923 MPa Tensile Strength 1659 MPa Tear Strength 1351 MPa Stretch LoadLimit 3.2% Elongation Breakage 5.0% Contraction at Fracture 18.3% 

It has been shown that by the inventive process a substantial increasein yield strength can be achieved. For user as auto body components themeasured reduction in tensile strength is insignificant, since thisplays only a subordinate role in the design of the construct. The breakelongation decreases only insubstantially from 5 to 4.1% as a result ofthe inventive process. In contrast, the reduction in tear sensitivityincreases in comparison to the contraction at fracture. Here also thecondition with subsequent annealing or tempering shows, with 29.8instead of 18.3%, a significant improvement in performance compared toonly heat treated and hardened condition.

The course of tensioning of the tensile samples of Components A or B isreproduced in the form of tension/elongation diagrams in FIG. 1 forComponent A and in FIG. 2 for Component B.

For the Component A significant yield points or elastic limits arerecognizable. The curves clearly deviate following the linear (Hookish)area, in a short almost horizontal progression. The subsequent long areaof the plastic deformation is likewise comparatively level.

In contrast to this, the curves in FIG. 2 for Component B show nodeflection and no recognizable elastic limit.

1. A process for producing a shaped part from a pre-form of hardenableand heat deformable steel sheet metal, by means of heat forming withcold deforming tools, in which by hot forming a hardened component isproduced, exhibiting martensitic and/or banitic microstructure, whereinthe hardened component is tempered subsequent to hot forming, so that ashaped component is produced, which in comparison to the hardenedcomponent condition, exhibits at least in areas a higher yield strengthand/or pronounced yield strength.
 2. The process according to claim 1,wherein temperature and duration of tempering are so selected, that theyield strength is increased by at least 20%.
 3. A process for producinga shaped part from a pre-form of hardenable and hot deformable steelsheet metal, including the steps: a) heating the preform to anaustenitising temperature; b) press hardenening in a cold tool as aresult of which, by quencing, at least partially a martensitic and/orbanitic microstructure is produced, c) wherein an annealing of theshaped pre-form at temperatures below 400° C. with formation of a shapedpart with increased yield strength and/or break elongation occurs incomparison to the hardened component.
 4. The process according to claim1, wherein the pre-form is a chrome-molybdenum steel.
 5. The processaccording to claim 3, wherein the chrome-molybdenum steel is 25CrMo4,34CrMo4 or 42CrMo4.
 6. The process according to claim 1, wherein thepre-form is a boron alloyed case hardened and heat treated steel.
 7. Theprocess according to claim 7, wherein the boron alloyed case hardenedand heat treated steel is 17MnB3, 22MnB5 or 27MnCr5-2.
 8. The processaccording to claim 1, wherein the pre-form is a flat plate, a colddeformed steel pre-form or cut or trend pre-form.
 9. The processaccording to claim 1, wherein the annealing occurs at a temperature inthe range of 250 to 400° C.
 10. The process according to claim 1,wherein the annealing occurs at a temperature of from 300 to 330° C. 11.The process according to claim 1, wherein the hardened pre-form is cutto its final contour prior to or subsequent to step c).
 12. The processaccording to claim 1, wherein the surface of the hardened shaped part iscleansed prior to annealing, in particular, by a powder blastingprocess.
 13. The process according to claim 1, wherein during annealingin step c) at the same time also a surface treatment for formation of adefined corrosion protection and/or friction resistance for the shapedpart occurs.
 14. The process according to claim 13, wherein the surfacetreatment is galvanizing or thin layer zinking.
 15. The processaccording to claim 1, wherein the pre-form is a collection of multipleheat deformable steel sheets, of which at least one is comprised of ahardenable steel.
 16. The process according to claim 1, wherein theshaped part is used to produce shell parts for the hollow beams orcarriers integrated in the passenger cell of a vehicle undercarriage.17. The process according to claim 1, wherein from the shaped part shellparts are produced for a hollow beam integrated in the undercarriage ofa vehicle.